Available Fault Current (AFC) calculations are most often performed by using fault current calculating software or spreadsheets that automatically calculate the final amount of AFC after entering specific values. Long before the software became available, the calculation was done manually with a few basic formulas that are commonly found in many of the electrician’s pocket guides like the “Ugly’s Electrical References” guide. These types of guides provide the most common electrical references and formulas used by the electrician. It is important to understand how the basic formula works even if the plan is to use software for the calculation.

Below is a 3-step formula to calculate three-phase AFC, also called the available short circuit current (ISC) at the end of a run of wire:

Step 1:

 F = (1.732 X L X I) ÷ (C X E_(L_L))

Step 2:

 Multiplier (M) = 1 ÷ (1 + F)

Step 3:

 Available Short-Circuit Current (ISC) = I X M

Here is an example using realistic data for the below electrical installation:

• 3 conductors in a raceway
• 3/0 Copper conductors
• PVC conduit
• 3-phase 480 Volt system
• AFC 35,000 amps
• Length of conductor 95 feet

Step 1:

F = (1.732 X 95 X 35,000) ÷ (13,923 X 480)     F = .861

Step 2:

Multiplier (M) = 1 ÷ (1 + .861)     M = .537

Step 3:

ISC = 35,000 X .537 = 18,795 amps (18,795 is the available fault current at the end of the run of conductor).